author | paulson |
Fri, 18 Oct 1996 11:41:04 +0200 | |
changeset 2107 | 23e8f15ec95f |
parent 2052 | d9f7f4b2613e |
child 2131 | 3106a99d30a5 |
permissions | -rw-r--r-- |
2002 | 1 |
(* Title: HOL/Auth/OtwayRees_Bad |
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ID: $Id$ |
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Author: Lawrence C Paulson, Cambridge University Computer Laboratory |
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Copyright 1996 University of Cambridge |
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Inductive relation "otway" for the Otway-Rees protocol. |
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The FAULTY version omitting encryption of Nonce NB, as suggested on page 247 of |
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Burrows, Abadi and Needham. A Logic of Authentication. |
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Proc. Royal Soc. 426 (1989) |
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This file illustrates the consequences of such errors. We can still prove |
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impressive-looking properties such as Spy_not_see_encrypted_key, yet the |
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protocol is open to a middleperson attack. Attempting to prove some key lemmas |
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indicates the possibility of this attack. |
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*) |
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open OtwayRees_Bad; |
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proof_timing:=true; |
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HOL_quantifiers := false; |
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(*Weak liveness: there are traces that reach the end*) |
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goal thy |
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"!!A B. [| A ~= B; A ~= Server; B ~= Server |] \ |
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\ ==> EX K. EX NA. EX evs: otway. \ |
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\ Says B A {|Nonce NA, Crypt {|Nonce NA, Key K|} (shrK A)|} \ |
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\ : set_of_list evs"; |
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by (REPEAT (resolve_tac [exI,bexI] 1)); |
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by (rtac (otway.Nil RS otway.OR1 RS otway.OR2 RS otway.OR3 RS otway.OR4) 2); |
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by (ALLGOALS (simp_tac (!simpset setsolver safe_solver))); |
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by (REPEAT_FIRST (resolve_tac [refl, conjI])); |
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by (ALLGOALS (fast_tac (!claset addss (!simpset setsolver safe_solver)))); |
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result(); |
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(**** Inductive proofs about otway ****) |
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(*The Spy can see more than anybody else, except for their initial state*) |
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goal thy |
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"!!evs. evs : otway ==> \ |
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2052 | 43 |
\ sees lost A evs <= initState lost A Un sees lost Spy evs"; |
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by (etac otway.induct 1); |
2002 | 45 |
by (ALLGOALS (fast_tac (!claset addDs [sees_Says_subset_insert RS subsetD] |
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addss (!simpset)))); |
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qed "sees_agent_subset_sees_Spy"; |
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2002 | 48 |
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(*Nobody sends themselves messages*) |
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goal thy "!!evs. evs : otway ==> ALL A X. Says A A X ~: set_of_list evs"; |
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by (etac otway.induct 1); |
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by (Auto_tac()); |
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qed_spec_mp "not_Says_to_self"; |
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Addsimps [not_Says_to_self]; |
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AddSEs [not_Says_to_self RSN (2, rev_notE)]; |
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(** For reasoning about the encrypted portion of messages **) |
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goal thy "!!evs. Says A' B {|N, Agent A, Agent B, X|} : set_of_list evs ==> \ |
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\ X : analz (sees lost Spy evs)"; |
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by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]) 1); |
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qed "OR2_analz_sees_Spy"; |
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2002 | 65 |
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goal thy "!!evs. Says S B {|N, X, X'|} : set_of_list evs ==> \ |
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\ X : analz (sees lost Spy evs)"; |
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by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj]) 1); |
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qed "OR4_analz_sees_Spy"; |
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goal thy "!!evs. Says B' A {|N, Crypt {|N,K|} K'|} : set_of_list evs ==> \ |
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\ K : parts (sees lost Spy evs)"; |
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by (fast_tac (!claset addSEs partsEs |
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addSDs [Says_imp_sees_Spy RS parts.Inj]) 1); |
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qed "Reveal_parts_sees_Spy"; |
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(*OR2_analz... and OR4_analz... let us treat those cases using the same |
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argument as for the Fake case. This is possible for most, but not all, |
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proofs: Fake does not invent new nonces (as in OR2), and of course Fake |
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messages originate from the Spy. *) |
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bind_thm ("OR2_parts_sees_Spy", |
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OR2_analz_sees_Spy RS (impOfSubs analz_subset_parts)); |
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bind_thm ("OR4_parts_sees_Spy", |
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OR4_analz_sees_Spy RS (impOfSubs analz_subset_parts)); |
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||
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val parts_Fake_tac = |
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forward_tac [OR2_parts_sees_Spy] 4 THEN |
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forward_tac [OR4_parts_sees_Spy] 6 THEN |
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forward_tac [Reveal_parts_sees_Spy] 7; |
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(** Theorems of the form X ~: parts (sees lost Spy evs) imply that NOBODY |
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sends messages containing X! **) |
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(*Spy never sees another agent's shared key! (unless it is leaked at start)*) |
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goal thy |
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"!!evs. [| evs : otway; A ~: lost |] \ |
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\ ==> Key (shrK A) ~: parts (sees lost Spy evs)"; |
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by (etac otway.induct 1); |
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by parts_Fake_tac; |
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by (Auto_tac()); |
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(*Deals with Fake message*) |
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by (best_tac (!claset addDs [impOfSubs analz_subset_parts, |
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impOfSubs Fake_parts_insert]) 1); |
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qed "Spy_not_see_shrK"; |
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2002 | 107 |
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bind_thm ("Spy_not_analz_shrK", |
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[analz_subset_parts, Spy_not_see_shrK] MRS contra_subsetD); |
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Addsimps [Spy_not_see_shrK, Spy_not_analz_shrK]; |
2002 | 112 |
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(*We go to some trouble to preserve R in the 3rd and 4th subgoals |
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As usual fast_tac cannot be used because it uses the equalities too soon*) |
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val major::prems = |
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goal thy "[| Key (shrK A) : parts (sees lost Spy evs); \ |
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\ evs : otway; \ |
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\ A:lost ==> R \ |
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\ |] ==> R"; |
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by (rtac ccontr 1); |
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by (rtac ([major, Spy_not_see_shrK] MRS rev_notE) 1); |
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2002 | 122 |
by (swap_res_tac prems 2); |
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by (ALLGOALS (fast_tac (!claset addIs prems))); |
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qed "Spy_see_shrK_E"; |
2002 | 125 |
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bind_thm ("Spy_analz_shrK_E", |
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analz_subset_parts RS subsetD RS Spy_see_shrK_E); |
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2002 | 128 |
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AddSEs [Spy_see_shrK_E, Spy_analz_shrK_E]; |
2002 | 130 |
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(*** Future keys can't be seen or used! ***) |
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(*Nobody can have SEEN keys that will be generated in the future. |
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This has to be proved anew for each protocol description, |
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but should go by similar reasoning every time. Hardest case is the |
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standard Fake rule. |
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The Union over C is essential for the induction! *) |
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goal thy "!!evs. evs : otway ==> \ |
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\ length evs <= length evs' --> \ |
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\ Key (newK evs') ~: (UN C. parts (sees lost C evs))"; |
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by (etac otway.induct 1); |
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by parts_Fake_tac; |
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(*auto_tac does not work here, as it performs safe_tac first*) |
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by (ALLGOALS Asm_simp_tac); |
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by (REPEAT_FIRST (best_tac (!claset addDs [impOfSubs analz_subset_parts, |
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impOfSubs parts_insert_subset_Un, |
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Suc_leD] |
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addss (!simpset)))); |
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val lemma = result(); |
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(*Variant needed for the main theorem below*) |
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goal thy |
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"!!evs. [| evs : otway; length evs <= length evs' |] \ |
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\ ==> Key (newK evs') ~: parts (sees lost C evs)"; |
2002 | 156 |
by (fast_tac (!claset addDs [lemma]) 1); |
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qed "new_keys_not_seen"; |
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Addsimps [new_keys_not_seen]; |
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(*Another variant: old messages must contain old keys!*) |
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goal thy |
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"!!evs. [| Says A B X : set_of_list evs; \ |
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\ Key (newK evt) : parts {X}; \ |
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\ evs : otway \ |
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\ |] ==> length evt < length evs"; |
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by (rtac ccontr 1); |
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by (dtac leI 1); |
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by (fast_tac (!claset addSDs [new_keys_not_seen, Says_imp_sees_Spy] |
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addIs [impOfSubs parts_mono]) 1); |
2002 | 170 |
qed "Says_imp_old_keys"; |
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(*** Future nonces can't be seen or used! [proofs resemble those above] ***) |
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goal thy "!!evs. evs : otway ==> \ |
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\ length evs <= length evs' --> \ |
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\ Nonce (newN evs') ~: (UN C. parts (sees lost C evs))"; |
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by (etac otway.induct 1); |
2002 | 179 |
(*auto_tac does not work here, as it performs safe_tac first*) |
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by (ALLGOALS (asm_simp_tac (!simpset addsimps [parts_insert2] |
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addcongs [disj_cong]))); |
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by (REPEAT_FIRST (fast_tac (!claset |
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addSEs partsEs |
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addSDs [Says_imp_sees_Spy RS parts.Inj] |
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addDs [impOfSubs analz_subset_parts, |
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impOfSubs parts_insert_subset_Un, |
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Suc_leD] |
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addss (!simpset)))); |
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2002 | 189 |
val lemma = result(); |
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(*Variant needed for the main theorem below*) |
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goal thy |
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"!!evs. [| evs : otway; length evs <= length evs' |] \ |
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\ ==> Nonce (newN evs') ~: parts (sees lost C evs)"; |
2002 | 195 |
by (fast_tac (!claset addDs [lemma]) 1); |
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qed "new_nonces_not_seen"; |
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Addsimps [new_nonces_not_seen]; |
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(*Another variant: old messages must contain old nonces!*) |
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goal thy |
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"!!evs. [| Says A B X : set_of_list evs; \ |
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\ Nonce (newN evt) : parts {X}; \ |
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\ evs : otway \ |
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\ |] ==> length evt < length evs"; |
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by (rtac ccontr 1); |
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by (dtac leI 1); |
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by (fast_tac (!claset addSDs [new_nonces_not_seen, Says_imp_sees_Spy] |
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addIs [impOfSubs parts_mono]) 1); |
2002 | 209 |
qed "Says_imp_old_nonces"; |
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(*Nobody can have USED keys that will be generated in the future. |
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...very like new_keys_not_seen*) |
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goal thy "!!evs. evs : otway ==> \ |
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\ length evs <= length evs' --> \ |
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\ newK evs' ~: keysFor (UN C. parts (sees lost C evs))"; |
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by (etac otway.induct 1); |
2002 | 218 |
by parts_Fake_tac; |
219 |
by (ALLGOALS Asm_simp_tac); |
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(*OR1 and OR3*) |
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by (EVERY (map (fast_tac (!claset addDs [Suc_leD] addss (!simpset))) [4,2])); |
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(*Fake, OR2, OR4: these messages send unknown (X) components*) |
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by (REPEAT |
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(best_tac |
2002 | 225 |
(!claset addDs [impOfSubs (analz_subset_parts RS keysFor_mono), |
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impOfSubs (parts_insert_subset_Un RS keysFor_mono), |
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Suc_leD] |
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addEs [new_keys_not_seen RS not_parts_not_analz RSN(2,rev_notE)] |
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addss (!simpset)) 1)); |
2002 | 230 |
val lemma = result(); |
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goal thy |
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"!!evs. [| evs : otway; length evs <= length evs' |] \ |
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\ ==> newK evs' ~: keysFor (parts (sees lost C evs))"; |
2002 | 235 |
by (fast_tac (!claset addSDs [lemma] addss (!simpset)) 1); |
236 |
qed "new_keys_not_used"; |
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bind_thm ("new_keys_not_analzd", |
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[analz_subset_parts RS keysFor_mono, |
240 |
new_keys_not_used] MRS contra_subsetD); |
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2002 | 241 |
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Addsimps [new_keys_not_used, new_keys_not_analzd]; |
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244 |
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(** Lemmas concerning the form of items passed in messages **) |
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246 |
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247 |
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248 |
(**** |
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The following is to prove theorems of the form |
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250 |
||
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Key K : analz (insert (Key (newK evt)) (sees lost Spy evs)) ==> |
252 |
Key K : analz (sees lost Spy evs) |
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2002 | 253 |
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A more general formula must be proved inductively. |
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255 |
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256 |
****) |
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257 |
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258 |
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(** Session keys are not used to encrypt other session keys **) |
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260 |
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(*Describes the form of Key K when the following message is sent. The use of |
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"parts" strengthens the induction hyp for proving the Fake case. The |
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assumptions on A are needed to prevent its being a Faked message. (Based |
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on NS_Shared/Says_S_message_form) *) |
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265 |
goal thy |
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"!!evs. evs: otway ==> \ |
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\ Crypt {|N, Key K|} (shrK A) : parts (sees lost Spy evs) & \ |
268 |
\ A ~: lost --> \ |
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2002 | 269 |
\ (EX evt:otway. K = newK evt)"; |
2032 | 270 |
by (etac otway.induct 1); |
2002 | 271 |
by parts_Fake_tac; |
272 |
by (Auto_tac()); |
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(*Deals with Fake message*) |
|
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by (best_tac (!claset addDs [impOfSubs analz_subset_parts, |
|
2032 | 275 |
impOfSubs Fake_parts_insert]) 1); |
2002 | 276 |
val lemma = result() RS mp; |
277 |
||
278 |
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(*EITHER describes the form of Key K when the following message is sent, |
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OR reduces it to the Fake case.*) |
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281 |
goal thy |
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282 |
"!!evs. [| Says B' A {|N, Crypt {|N, Key K|} (shrK A)|} : set_of_list evs; \ |
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\ evs : otway |] \ |
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2052 | 284 |
\ ==> (EX evt:otway. K = newK evt) | Key K : analz (sees lost Spy evs)"; |
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285 |
by (case_tac "A : lost" 1); |
2032 | 286 |
by (fast_tac (!claset addSDs [Says_imp_sees_Spy RS analz.Inj] |
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addss (!simpset)) 1); |
2002 | 288 |
by (forward_tac [lemma] 1); |
289 |
by (fast_tac (!claset addEs partsEs |
|
2032 | 290 |
addSDs [Says_imp_sees_Spy RS parts.Inj]) 1); |
2002 | 291 |
by (Fast_tac 1); |
292 |
qed "Reveal_message_form"; |
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293 |
||
294 |
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295 |
(*Lemma for the trivial direction of the if-and-only-if*) |
|
296 |
goal thy |
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297 |
"!!evs. (Key K : analz (Key``nE Un sEe)) --> \ |
|
298 |
\ (K : nE | Key K : analz sEe) ==> \ |
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299 |
\ (Key K : analz (Key``nE Un sEe)) = (K : nE | Key K : analz sEe)"; |
|
300 |
by (fast_tac (!claset addSEs [impOfSubs analz_mono]) 1); |
|
301 |
val lemma = result(); |
|
302 |
||
303 |
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304 |
(*The equality makes the induction hypothesis easier to apply*) |
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305 |
goal thy |
|
306 |
"!!evs. evs : otway ==> \ |
|
2052 | 307 |
\ ALL K E. (Key K : analz (Key``(newK``E) Un (sees lost Spy evs))) = \ |
308 |
\ (K : newK``E | Key K : analz (sees lost Spy evs))"; |
|
2032 | 309 |
by (etac otway.induct 1); |
310 |
by (dtac OR2_analz_sees_Spy 4); |
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311 |
by (dtac OR4_analz_sees_Spy 6); |
|
312 |
by (dtac Reveal_message_form 7); |
|
2002 | 313 |
by (REPEAT_FIRST (ares_tac [allI, lemma])); |
314 |
by (REPEAT ((eresolve_tac [bexE, disjE] ORELSE' hyp_subst_tac) 7)); |
|
315 |
by (ALLGOALS (*Takes 28 secs*) |
|
316 |
(asm_simp_tac |
|
317 |
(!simpset addsimps ([insert_Key_singleton, insert_Key_image, pushKey_newK] |
|
2032 | 318 |
@ pushes) |
2002 | 319 |
setloop split_tac [expand_if]))); |
320 |
(** LEVEL 7 **) |
|
321 |
(*Reveal case 2, OR4, OR2, Fake*) |
|
2032 | 322 |
by (EVERY (map spy_analz_tac [7,5,3,2])); |
2002 | 323 |
(*Reveal case 1, OR3, Base*) |
324 |
by (Auto_tac()); |
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325 |
qed_spec_mp "analz_image_newK"; |
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326 |
||
327 |
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328 |
goal thy |
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329 |
"!!evs. evs : otway ==> \ |
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2052 | 330 |
\ Key K : analz (insert (Key (newK evt)) (sees lost Spy evs)) = \ |
331 |
\ (K = newK evt | Key K : analz (sees lost Spy evs))"; |
|
2002 | 332 |
by (asm_simp_tac (HOL_ss addsimps [pushKey_newK, analz_image_newK, |
2032 | 333 |
insert_Key_singleton]) 1); |
2002 | 334 |
by (Fast_tac 1); |
335 |
qed "analz_insert_Key_newK"; |
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336 |
||
337 |
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338 |
(*Describes the form of K and NA when the Server sends this message.*) |
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339 |
goal thy |
|
340 |
"!!evs. [| Says Server B \ |
|
341 |
\ {|NA, Crypt {|NA, K|} (shrK A), \ |
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342 |
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs; \ |
|
343 |
\ evs : otway |] \ |
|
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\ ==> (EX evt:otway. K = Key(newK evt)) & \ |
|
2052 | 345 |
\ (EX i. NA = Nonce i) & \ |
346 |
\ (EX j. NB = Nonce j)"; |
|
2032 | 347 |
by (etac rev_mp 1); |
348 |
by (etac otway.induct 1); |
|
2002 | 349 |
by (ALLGOALS (fast_tac (!claset addIs [less_SucI] addss (!simpset)))); |
350 |
qed "Says_Server_message_form"; |
|
351 |
||
352 |
||
353 |
(*Crucial security property, but not itself enough to guarantee correctness! |
|
354 |
The need for quantification over N, C seems to indicate the problem. |
|
355 |
Omitting the Reveal message from the description deprives us of even |
|
2032 | 356 |
this clue. *) |
2002 | 357 |
goal thy |
2052 | 358 |
"!!evs. [| A ~: lost; B ~: lost; evs : otway; evt : otway |] \ |
2002 | 359 |
\ ==> Says Server B \ |
2052 | 360 |
\ {|NA, Crypt {|NA, Key K|} (shrK A), \ |
361 |
\ Crypt {|NB, Key K|} (shrK B)|} : set_of_list evs --> \ |
|
362 |
\ (ALL N C. Says C Spy {|N, Key K|} ~: set_of_list evs) --> \ |
|
363 |
\ Key K ~: analz (sees lost Spy evs)"; |
|
2032 | 364 |
by (etac otway.induct 1); |
365 |
by (dtac OR2_analz_sees_Spy 4); |
|
366 |
by (dtac OR4_analz_sees_Spy 6); |
|
367 |
by (dtac Reveal_message_form 7); |
|
2002 | 368 |
by (REPEAT_FIRST (eresolve_tac [asm_rl, bexE, disjE] ORELSE' hyp_subst_tac)); |
369 |
by (ALLGOALS |
|
370 |
(asm_full_simp_tac |
|
371 |
(!simpset addsimps ([analz_subset_parts RS contra_subsetD, |
|
2032 | 372 |
analz_insert_Key_newK] @ pushes) |
2002 | 373 |
setloop split_tac [expand_if]))); |
374 |
(** LEVEL 6 **) |
|
375 |
(*Reveal case 1*) |
|
376 |
by (Fast_tac 5); |
|
377 |
(*OR3*) |
|
378 |
by (fast_tac (!claset addSIs [parts_insertI] |
|
2032 | 379 |
addEs [Says_imp_old_keys RS less_irrefl] |
2052 | 380 |
addss (!simpset addsimps [parts_insert2])) 3); |
2002 | 381 |
(*Reveal case 2, OR4, OR2, Fake*) |
2032 | 382 |
by (rtac conjI 3); |
383 |
by (REPEAT (spy_analz_tac 1)); |
|
2002 | 384 |
val lemma = result() RS mp RS mp RSN(2,rev_notE); |
385 |
||
386 |
||
387 |
||
388 |
(*WEAK VERSION: NEED TO ELIMINATE QUANTIFICATION OVER N, C!!*) |
|
389 |
goal thy |
|
390 |
"!!evs. [| Says Server B \ |
|
391 |
\ {|NA, Crypt {|NA, K|} (shrK A), \ |
|
392 |
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs; \ |
|
2032 | 393 |
\ (ALL N C. Says C Spy {|N, K|} ~: set_of_list evs); \ |
2052 | 394 |
\ A ~: lost; B ~: lost; evs : otway |] \ |
395 |
\ ==> K ~: analz (sees lost Spy evs)"; |
|
2002 | 396 |
by (forward_tac [Says_Server_message_form] 1 THEN assume_tac 1); |
397 |
by (fast_tac (!claset addSEs [lemma]) 1); |
|
2032 | 398 |
qed "Spy_not_see_encrypted_key"; |
2002 | 399 |
|
400 |
||
401 |
(*** Attempting to prove stronger properties ***) |
|
402 |
||
403 |
(** The Key K uniquely identifies the Server's message. **) |
|
404 |
||
405 |
fun ex_strip_tac i = REPEAT (ares_tac [exI, conjI] i) THEN assume_tac (i+1); |
|
406 |
||
407 |
goal thy |
|
408 |
"!!evs. evs : otway ==> \ |
|
409 |
\ EX A' B' NA' NB'. ALL A B NA NB. \ |
|
410 |
\ Says Server B \ |
|
411 |
\ {|NA, Crypt {|NA, K|} (shrK A), \ |
|
412 |
\ Crypt {|NB, K|} (shrK B)|} : set_of_list evs --> \ |
|
413 |
\ A=A' & B=B' & NA=NA' & NB=NB'"; |
|
2032 | 414 |
by (etac otway.induct 1); |
2002 | 415 |
by (ALLGOALS (asm_simp_tac (!simpset addsimps [all_conj_distrib]))); |
416 |
by (Step_tac 1); |
|
417 |
(*Remaining cases: OR3 and OR4*) |
|
418 |
by (ex_strip_tac 2); |
|
419 |
by (Fast_tac 2); |
|
2107
23e8f15ec95f
The new proof of the lemma for new_nonces_not_seen is faster
paulson
parents:
2052
diff
changeset
|
420 |
by (expand_case_tac "K = ?y" 1); |
23e8f15ec95f
The new proof of the lemma for new_nonces_not_seen is faster
paulson
parents:
2052
diff
changeset
|
421 |
by (REPEAT (ares_tac [refl,exI,impI,conjI] 2)); |
2002 | 422 |
(*...we assume X is a very new message, and handle this case by contradiction*) |
423 |
by (fast_tac (!claset addEs [Says_imp_old_keys RS less_irrefl] |
|
2032 | 424 |
delrules [conjI] (*prevent split-up into 4 subgoals*) |
425 |
addss (!simpset addsimps [parts_insertI])) 1); |
|
2002 | 426 |
val lemma = result(); |
427 |
||
428 |
goal thy |
|
429 |
"!!evs. [| Says Server B \ |
|
430 |
\ {|NA, Crypt {|NA, K|} (shrK A), \ |
|
431 |
\ Crypt {|NB, K|} (shrK B)|} \ |
|
432 |
\ : set_of_list evs; \ |
|
433 |
\ Says Server B' \ |
|
434 |
\ {|NA', Crypt {|NA', K|} (shrK A'), \ |
|
435 |
\ Crypt {|NB', K|} (shrK B')|} \ |
|
436 |
\ : set_of_list evs; \ |
|
437 |
\ evs : otway |] \ |
|
438 |
\ ==> A=A' & B=B' & NA=NA' & NB=NB'"; |
|
2032 | 439 |
by (dtac lemma 1); |
2002 | 440 |
by (REPEAT (etac exE 1)); |
441 |
(*Duplicate the assumption*) |
|
442 |
by (forw_inst_tac [("psi", "ALL C.?P(C)")] asm_rl 1); |
|
443 |
by (fast_tac (!claset addSDs [spec]) 1); |
|
444 |
qed "unique_session_keys"; |
|
445 |
||
446 |
||
2052 | 447 |
(*Only OR1 can have caused such a part of a message to appear. |
448 |
I'm not sure why A ~= B premise is needed: OtwayRees.ML doesn't need it. |
|
449 |
Perhaps it's because OR2 has two similar-looking encrypted messages in |
|
450 |
this version.*) |
|
2002 | 451 |
goal thy |
2052 | 452 |
"!!evs. [| A ~: lost; A ~= B; evs : otway |] \ |
2002 | 453 |
\ ==> Crypt {|NA, Agent A, Agent B|} (shrK A) \ |
2052 | 454 |
\ : parts (sees lost Spy evs) --> \ |
2002 | 455 |
\ Says A B {|NA, Agent A, Agent B, \ |
456 |
\ Crypt {|NA, Agent A, Agent B|} (shrK A)|} \ |
|
457 |
\ : set_of_list evs"; |
|
2032 | 458 |
by (etac otway.induct 1); |
2002 | 459 |
by parts_Fake_tac; |
460 |
by (ALLGOALS Asm_simp_tac); |
|
461 |
(*Fake*) |
|
462 |
by (best_tac (!claset addSDs [impOfSubs analz_subset_parts, |
|
2032 | 463 |
impOfSubs Fake_parts_insert]) 2); |
2002 | 464 |
by (Auto_tac()); |
465 |
qed_spec_mp "Crypt_imp_OR1"; |
|
466 |
||
467 |
||
468 |
(*This key property is FALSE. Somebody could make a fake message to Server |
|
469 |
substituting some other nonce NA' for NB.*) |
|
470 |
goal thy |
|
2052 | 471 |
"!!evs. [| A ~: lost; A ~= Spy; evs : otway |] \ |
472 |
\ ==> Crypt {|NA, Key K|} (shrK A) : parts (sees lost Spy evs) --> \ |
|
473 |
\ Says A B {|NA, Agent A, Agent B, \ |
|
474 |
\ Crypt {|NA, Agent A, Agent B|} (shrK A)|} \ |
|
2002 | 475 |
\ : set_of_list evs --> \ |
476 |
\ (EX B NB. Says Server B \ |
|
2052 | 477 |
\ {|NA, \ |
478 |
\ Crypt {|NA, Key K|} (shrK A), \ |
|
479 |
\ Crypt {|NB, Key K|} (shrK B)|} \ |
|
2002 | 480 |
\ : set_of_list evs)"; |
2032 | 481 |
by (etac otway.induct 1); |
2052 | 482 |
by parts_Fake_tac; |
2002 | 483 |
by (ALLGOALS Asm_simp_tac); |
484 |
(*Fake*) |
|
485 |
by (best_tac (!claset addSDs [impOfSubs analz_subset_parts, |
|
2032 | 486 |
impOfSubs Fake_parts_insert]) 1); |
2002 | 487 |
(*OR1: it cannot be a new Nonce, contradiction.*) |
488 |
by (fast_tac (!claset addSIs [parts_insertI] |
|
2032 | 489 |
addSEs partsEs |
490 |
addEs [Says_imp_old_nonces RS less_irrefl] |
|
491 |
addss (!simpset)) 1); |
|
2002 | 492 |
(*OR3 and OR4*) (** LEVEL 5 **) |
493 |
(*OR4*) |
|
494 |
by (REPEAT (Safe_step_tac 2)); |
|
2052 | 495 |
by (REPEAT (best_tac (!claset addSDs [parts_cut]) 3)); |
496 |
by (fast_tac (!claset addSIs [Crypt_imp_OR1] |
|
497 |
addEs partsEs |
|
498 |
addDs [Says_imp_sees_Spy RS parts.Inj]) 2); |
|
499 |
(*OR3*) (** LEVEL 8 **) |
|
2002 | 500 |
by (ALLGOALS (asm_simp_tac (!simpset addsimps [ex_disj_distrib]))); |
2052 | 501 |
by (step_tac (!claset delrules [disjCI, impCE]) 1); |
2002 | 502 |
(*The hypotheses at this point suggest an attack in which nonce NA is used |
2052 | 503 |
in two different roles: |
504 |
Says B' Server |
|
505 |
{|Nonce NAa, Agent Aa, Agent A, |
|
506 |
Crypt {|Nonce NAa, Agent Aa, Agent A|} (shrK Aa), Nonce NA, |
|
507 |
Crypt {|Nonce NAa, Agent Aa, Agent A|} (shrK A)|} |
|
508 |
: set_of_list evsa; |
|
509 |
Says A B |
|
510 |
{|Nonce NA, Agent A, Agent B, |
|
511 |
Crypt {|Nonce NA, Agent A, Agent B|} (shrK A)|} |
|
512 |
: set_of_list evsa |
|
513 |
*) |
|
2002 | 514 |
writeln "GIVE UP!"; |
515 |
||
516 |
||
2052 | 517 |
(*Thus the key property A_can_trust probably fails too.*) |